The present invention relates to microelectromechanical capacitor structures, and more particularly to variably controlled, electrostatically activated, high-voltage capacitor structures.
Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (Micro Electro Mechanical System) structures. MEMS structures are typically capable of motion or applying force. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size.
Design freedom afforded to engineers of MEMS devices has led to the development of various techniques and structures for providing the force necessary to cause the desired motion within microstructures. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have also been successfully been used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices. One such device is found in U.S. Pat. No. 5,475,318 entitled xe2x80x9cMicroprobexe2x80x9d issued Dec. 12, 1995 in the name of inventors Marcus et al., which leverages thermal expansion to move a microdevice. A micro cantilever is constructed from materials having different thermal coefficients of expansion. When heated, the bimorph layers arch differently, causing the micro cantilever to move accordingly. A similar mechanism is used to activate a micromachined thermal switch as described in U.S. Pat. No. 5,463,233 entitled xe2x80x9cMicromachined Thermal Switchxe2x80x9d issued Oct. 31, 1995 in the name of inventor Norling.
Electrostatic forces have also been used to move structures. Traditional electrostatic devices were constructed from laminated films cut from plastic or mylar materials. A flexible electrode was attached to the film, and another electrode was affixed to a base structure. Electrically energizing the respective electrodes created an electrostatic force attracting the electrodes to each other or repelling them from each other. A representative example of these devices is found in U.S. Pat. No. 4,266,339 entitled xe2x80x9cMethod for Making Rolling Electrode for Electrostatic Devicexe2x80x9d issued May 12, 1981 in the name of inventor Kalt. These devices work well for typical motive applications, but these devices cannot be constructed in dimensions suitable for miniaturized integrated circuits, biomedical applications, or MEMS structures.
MEMS electrostatic capacitors are used advantageously in various applications because of their extremely small size. Electrostatic forces due to the electric field between electrical charges can generate relatively large forces given the small electrode separations inherent in MEMS devices. However, problems may arise when these miniaturized devices are used in high voltage applications. Because MEMS devices include structures separated by micron scale dimensions, high voltages can create electrical arcing and other related problems. In typical MEMS devices, the air gap separation between the substrate electrode and moveable composite electrode affects the electrostatic voltage required to move the composite electrode and operate the device. A relatively large air gap is beneficial for minimizing high voltage problems. However, the larger the air gaps, the higher the voltage required to operate the electrostatic MEMS device. As such, traditional MEMS electrostatic devices are not well suited for high voltage switching applications.
It would be advantageous to form high voltage capacitors using MEMS devices operable with relatively low electrostatic voltages. Additionally, it would advantageous to provide for a MEMS structure having a larger air gap separation between the substrate electrode and the moveable composite electrode than has been previously exhibited in MEMS capacitors. The larger air gap would allow for very small capacitance for a given area and, hence, a variable capacitor over an extended range. It would also be beneficial to provide for a MEMS capacitor that minimizes the occurrence of stiction between the substrate and moveable composite. Stiction, which is a well-known concept in microelectronics, is defined as the tendency for contacting MEMS surfaces to stick to one another. In addition, it would be advantageous to develop a MEMS variable capacitor that exhibits complete electrical isolation between the capacitor plates. Furthermore, it would be advantageous to provide a variably controlled MEMS electrostatic capacitor that overcomes at least some of the arcing and high voltage operational problems attributed to typical MEMS devices. There is still a need to develop improved MEMS devices for variable controlled capacitors while leveraging electrostatic forces. In addition, leveraging the electrostatic forces in new variable controlled MEMS capacitors could create advantageous new devices and applications.
The present invention provides for improved MEMS electrostatic devices that can operate as high voltage, variable controlled capacitors. Further, a method for using and a method for making the MEMS electrostatic device according to the present invention are provided.
A MEMS device driven by electrostatic forces according to the present invention comprises a microelectronic substrate, a substrate signal electrode, and one or more substrate control electrodes. Further, the MEMS device of the present invention includes a moveable composite having a composite signal electrode, one or more substrate control electrodes and a biasing element. Additionally, insulators are provided to insure electrical isolation between the electrodes. A microelectronic substrate defines a generally planar surface upon which the MEMS device is constructed. The substrate signal electrode and the substrate control electrode form at least one layer on the surface of the microelectronic substrate. The moveable composite is attached to the substrate construct and overlies the substrate electrodes. In cross section, the moveable composite generally comprises one or more electrode layers and, in most embodiments, a biasing layer. The moveable composite across its length comprises a fixed portion attached to the underlying substrate, a medial portion adjacent to the fixed portion and a distal portion adjacent to the medial portion. The medial and distal portions being moveable with respect to the substrate electrode. Applying a voltage between the substrate control electrode and moveable composite control electrode creates an electrostatic force that attracts the moveable distal portion of the composite to the underlying microelectronic substrate and controls biasing. As such, the resulting capacitance between the substrate signal electrode and the moveable composite signal electrode is effectively controlled.
One embodiment of the MEMS electrostatic device according to the present invention forms one or more layers of the moveable composite from one or more generally flexible materials. Layers comprising the composite can be selected such that the moveable composite substantially conforms to the surface of the microelectronic substrate when the distal portion of the moveable composite is attracted to the microelectronic substrate. In addition, layers comprising the composite can be selected such that the distal portion can be positionally biased with respect to the microelectronic substrate when no electrostatic force is applied.
Alternatively, another embodiment of the present invention allows for the relative area and the shape of the control electrodes to be adjusted so as to control the bias between the moveable composite while maintaining constant voltage. Additionally, another embodiment of the present invention allows for the surface area of the moveable composite to be generally fully covered with a combination of signal and control electrodes for the purpose of insuring uniformity and consistency in the movement of the composite.
In addition, another embodiment of the present invention provides a method of using the electrostatic MEMS devices described above. The method comprises the step of selectively generating a variable electrostatic force between the substrate control electrode and the composite control electrode. Further, the method comprises moving the moveable composite toward the substrate in response to the variable electrostatic force to vary the capacitance between the substrate signal electrode and the composite signal electrode.
In yet another embodiment of the present invention, a method is provided for making the electrostatic MEMS device described above. The method comprises the steps of selecting a microelectronic substrate, forming a substrate signal electrode within layer on the planar surface of the substrate, forming a substrate control electrode within a layer on the planar surface of the substrate and forming a release layer over a portion of the uppermost surface of the substrate construct. Additionally, the method comprises the steps of attaching a moveable composite having a signal electrode, control electrode and a biasing element to the uppermost surface of the substrate construct and removing the release layer to create an air gap between the substrate construct and the moveable composite.
The moveable composite design of the MEMS high voltage capacitors of the present invention are operable with relatively low electrostatic voltages. Additionally, the large air gap separation between the moveable composite and the substrate results in very small capacitance for a given area and, therefore, variable capacitance can be achieved over a larger range than previous MEMS variable capacitors. Additional benefit is realized by the variable capacitor of the present invention in that the curled nature of the medial and distal portions of the moveable composite lessen the occurrence of stiction resulting between the substrate and the moveable composite. The variable capacitor of the present invention is able to provide complete electrical isolation between the control and signal electrodes by providing for the use of flexible insulting polymer films in the moveable composite. As such, MEMS variably controlled capacitors that have these improved performance characteristics, and many others that will be readily apparent to those of ordinary skill in the art, are desired for many new microelectronic devices and applications.